This invention relates to traveling wave optical modulators and more specifically to a design for such a modulator which provides a substantial improvement in performance. A discussion of conventional optical modulators can be found in "Integrated Optical Circuits and Components, Design and Applications," edited by Lynn D. Hutcheson, published by Marcel Dekker, Inc., 1987. See Chapter 6 entitled "Ti:LiNbO3 Integrated Optic Technology."
Optical modulators capable of modulating the intensity of light efficiently at high frequencies are valuable in the area of fiber optic telecommunications. Because of the very large bandwidth achievable by optical transmission, and because much of existing communications technology uses electrical rather than optical means to implement functions such as encoding information on an optical carrier, it is desirable to mate the electrical and optical technologies. A device such as a traveling wave optical modulator which provides an efficient way to intensity-modulate light at high frequency by means of an electrical signal accomplishes this.
Traveling wave optical modulators use an electromagnetic signal such as a microwave signal to intensity-modulate light in an optical waveguide channel. Typically, crystal material such as lithium niobate is used as a substrate within which the optical waveguide channel is formed. The optical waveguide channel is formed by photolithographic masking and diffusion of an element such as titanium into the crystal substrate to produce a region with a higher optical index of refraction than the optical index of refraction of the surrounding substrate. The difference in refractive indices produces total internal reflection to thereby constrain the propagation of light within the optical waveguide channel.
The optical index of refraction of a material relates to the phase velocity of light in the material. The crystal substrate has the property that its optical index of refraction is affected by an electromagnetic signal. The electromagnetic signal is applied by means of electrodes. By placing the electromagnetic signal in close proximity to the optical waveguide channel such that the optical index of refraction of the waveguide in the substrate is affected, the light propagating along the waveguide may be modulated in response to the electromagnetic signal.
In an optical modulator where the optical waveguide channel is diffused into the substrate through one surface, the waveguide will be at that one surface of the substrate, and the electrodes for applying the electromagnetic signal are mounted on that one surface of the modulator. Usually the electrodes are elongated and are positioned parallel to the optical waveguide channel, with the waveguide being located either directly under or between the electrodes.
The optical waveguide channel is positioned to take advantage of the electric field's maximum effect on changing the optical index of refraction within the waveguide. Where an optical waveguide channel is diffused into a crystal substrate that exhibits more than one index of refraction in differing directions, it is preferable to utilize the index of refraction in the direction which is most susceptible to change by an electric field.
The primary design goals in fabricating traveling wave optical modulators are (1) broad modulation bandwidth, (2) low switching voltage and (3) impedance matching to the electromagnetic signal modulation source.
Broad modulation bandwidth is required to take advantage of the high-frequency capability of optical transmission. Typically, desired bandwidths have been on the order of tens of gigahertz. The bandwidth is limited primarily by the mismatch in velocity of the optical signal and the microwave signal used to modulate the optical signal. Since the microwave signal travels more slowly through the electrodes than the optical signal travels through the optical waveguide, a phase error is introduced. The extent of the phase error depends upon (1) the frequency of the microwave signal and (2) the length of interaction of the electric field generated by the microwave signal with the optical path.
The phase error decreases with a decrease in frequency of the microwave signal. However, the goal is generally to design a modulator which operates to as high a frequency as possible, so decreasing the frequency is not a design option. Making the length of interaction between the signals smaller would allow a higher bandwidth, but this also increases the switching voltage (discussed below).
It is therefore an object of the present invention to increase the bandwidth of the traveling wave optical modulator by improving velocity matching while maintaining a low switching voltage.
The switching voltage is defined to be a dc voltage which, when applied across the electrodes, turns the optical modulator from full off to full on. A low switching voltage is desired to optimize the amount of modulation for a given power level of the microwave modulating signal. A lower microwave power requirement allows for the design of less costly drive circuitry.
The primary parameters affecting the switching voltage are the electrode gap and electrode length. The smaller the distance or gap between the electrodes, the smaller will be the necessary switching voltage. But the electrodes cannot be placed too close together since the depth of electric field penetration into the substrate decreases as the gap size decreases. This means there will be less electric field overlap with the optical waveguide channel when the gap size is small. Also, as in some designs where two or more optical waveguide channels must be located directly below adjacent electrodes, the gap size is limited because the waveguides must not be too close together or cross-coupling between the waveguides will occur.
It is therefore another object of the present invention to achieve a low switching voltage while also achieving strong field overlap and preventing cross-coupling.
Finally, the output impedance from the microwave modulation source should ideally equal or "match" the input impedance of the optical modulator. An impedance mismatch causes power loss and reflections which could disrupt the drive circuitry. Typically, a microwave signal modulation source will have an impedance of 50 ohms which exceeds the impedance of the optical modulator. It is thus desirable to employ a design which increases optical modulator impedance.
The impedance of the optical modulator is primarily dependent upon the electrode gap and the "hot" electrode width. As used in this specification, a "hot" electrode is defined as a first electrode of an electrode pair where the second electrode of the pair is the ground electrode. The ground electrode is traditionally the electrode of greater width in the pair and is connected to the shielding means of a cable which provides a microwave signal to the optical modulator. A given electrode pair defines a gap width between the pair. A smaller gap creates a smaller impedance while a larger hot electrode width further reduces the impedance. As explained above, a small gap is desirable to reduce the switching voltage while a relatively large hot electrode width is desired to reduce electrode RF losses. This is a design conflict between impedance matching versus low switching voltage and low RF losses.
It is therefore another object of the present invention to achieve a low switching voltage while also achieving microwave impedance matching and low RF losses.
In summary, the design goals of a traveling wave optical modulator are broad modulation bandwidth, low switching voltage, and low microwave insertion loss which includes impedance matching. However, achieving these design goals has traditionally been complicated by the fact that in the design of an optical modulator, many of the parameters are interdependent upon and conflict with one another.